The topic of this proposal is the origin of paired helical filaments (PHF), a major pathological feature of Alzheimer's disease (AD). Although several lines of evidence point towards neuronal cytoskeletal proteins as possible constituents of PHF, their chemical composition remains uncertain. The task we now confront is to unravel the mechanisms that lead to the development of PHF and to ascertain their molecular origin. Our general hypothesis is that PHF are derived from neurofilament or tau proteins that are modified during the course of a disease, such as AD, or by aging. To explore this hypothesis, we propose the following strategies: . To study the in vitro assembly of normal neurofilament and tau proteins; . To submit these cytoskeletal proteins to chemical modifications in vitro and to observe the structural changes that ensue (i.e., formation of insoluble polymers or PHF). The effects of multivalent cations and partial proteolysis have been chosen as conditions that are relevant to posttranslational modifications that may lead to pathological changes of the cytoskeleton in vivo, such as occurs in AD; and . To compare the in vitro assembly of neurofilament and tau proteins isolated from the brains of individuals with AD to those of controls.